This simple model was shown to significantly alleviate the Hubble tension, with the introduction of a single low mass threshold, a “step” [1]}. It is easy to imagine extensions of this model, perhaps the simplest of which is if the relativistic fluid additionally has interactions with dark matter, \(\chi \) . If \(\chi \)  is a fermion, the simplest interaction possible is one in which an additional Yukawa coupling is added \(\phi \chi \chi \) . Interactions with the fluid would arise through Compton-like \(\phi \chi \rightarrow \phi \chi \)  processes, as well as \(\chi \psi \rightarrow \chi \psi \)  mediated by t-channel \(\phi \)  exchange.
